This is something I'm playing with and trying to figure out... I'm doing Rainbow Dash cosplay and given the fact the character is originally an animal-shaped-pony there is no official basis for what appropriate sizing would be for a more human-shaped version... Not sure whether to go for a "traditional" look with a much slimmer waist and much larger hips/bust, or a more minimal shape with smaller (but still noticeable) breasts/hips and a smaller hip:waist ratio... (from my limited knowledge of fitness, girls who exercise a lot (read professional runners (as close as I can equate to "professional flyer" like Dash)) tend to have smaller hips and breasts...

In the fandom I've seen drawings that go both ways and while I have my personal preference of which _I_ think looks better, it's just that //Personal Preference// so I'd like some feedback on what would be more realistic/expected (and pls no jokes about "go for anime double-E sized breasts" I am not doing that...(not to mention that would be outright ridiculous for the character))

In short: How much padding or what kind of hip:waist:bust ratio should I go for with a VERY athletic female character?

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'Bring peace to the lost lamb, grant rest to the fangs of the wolf, and deliver the Hammer of God unto the Devil', - Sister Rosette Christopher, Chrno Crusade

"The bad news is he ended up building the whole rock tower... the good news *gulp* I have a rock tower to knock down..." - Rainbow Dash, My Little Pony: Fiendship is Magic (S03E09)

I'd go for the athletic figure. I think that suits Rainbow Dash a lot better then an hour-glass shape with big boobs and broad hips.
For an athletic figure the bust and hip's should be about the same width. Say, if the bust is about 80 cm, the hips should be about 80 cm too. And the waist maybe 65 cm. That would give you a column body shape, which I think is the most common body shape for athletic women. I have a very athletic figure, and my measurements are 84-63-86, and I'm 172 cm tall. I hope this helps somewhat.. ^^

Your actually pretty close to my height (172cm = 5.6ft, so I'm guestimating around 5'7" or 5'8" and I'm 5'10") so I'll try adding a little to those dimensions later tonight and see how it turns out

One question though, the hip measurement does it include the buttocks? and if so should I pad those too? (I have a tiny rear so I suspect I should, which throws into question then if the buttocks and hips should be ~equal in size or if one should be larger than the other x_x I hate math in cosplay (lol))

__________________
'Bring peace to the lost lamb, grant rest to the fangs of the wolf, and deliver the Hammer of God unto the Devil', - Sister Rosette Christopher, Chrno Crusade

"The bad news is he ended up building the whole rock tower... the good news *gulp* I have a rock tower to knock down..." - Rainbow Dash, My Little Pony: Fiendship is Magic (S03E09)

Yes, when you measure the widest part of your hips it includes the buttocks. At least that is what I learned at school. This picture kind of shows where you have to measure. If you want to, you could pad the buttocks too, but it's not a must. But if you are wearing something that's very tight, you'd look more feminine if you padded the butt too.
At I con I went to last summer there was this guy who did an amazing crossplay of Bayonetta, which is an incredibly tight costume. And he had the most beautiful butt xD But he confessed that he had used some kind of padded panties like this. So that might be an idea.

Like you said, it's hard to translate ponies to human form. And there is a TON of variety in body type for athletes, even within the same sport. If you care to research athlete references you can try looking at the USA Olympic team roster: http://www.teamusa.org/Athletes.aspx?page=1 (you can filter by sport - since humans can't fly I would think soccer or swimming or volleyball would be well rounded picks).

If you want my opinion, the "flat chested runner" body type would most usually apply to young runners and serious marathon or Olympic grade athletes. More casual women runners are likely to just look like average women, but with toned/muscular thighs if you can actually see their legs. You certainly don't HAVE TO go with a typical feminine shape with wide hips and skinny waist, but that would still be acceptable for an athlete.

I'm going to go with a smi-middle-ground, since I have zero curves I think adding some noticeable but fairly minor ones would help the illusion (I've always said cosplay and especially crossplay is a game where you try to counter each "negative" trait with a positive one for a net-gain, so I think adding a few curves would help offset some of my un-hide-able male traits)

When I get my Zentai suit (probably next month sometime - low on $$$) I'll post some pictures of what I end up with

__________________
'Bring peace to the lost lamb, grant rest to the fangs of the wolf, and deliver the Hammer of God unto the Devil', - Sister Rosette Christopher, Chrno Crusade

"The bad news is he ended up building the whole rock tower... the good news *gulp* I have a rock tower to knock down..." - Rainbow Dash, My Little Pony: Fiendship is Magic (S03E09)